1. Field of the Invention
The present invention relates to the preparation of plastic articles by injection molding and gas-assisted injection molding techniques in which multiple cavities are simultaneously filled with plastic material. A new and improved method and apparatus for injection molding has been discovered whereby the amount of resin injected into each cavity of a multiple cavity mold or tool can be individually controlled. This system prevents either overfilling or underfilling of individual article cavities in such a multiple cavity tool and, therefore, allows for the production of more uniform plastic articles from a multiple cavity mold or tool.
2. Description of the Prior Art
Injection molding with multi-cavity molds has been used to produce multiple plastic articles simultaneously. These multiple articles can all be the same size and shape or they can be different sizes and shapes. In general, plastic material is fed to a single entry point or central sprue of the injection molding system from a plastic extrusion machine and then distributed to the various article cavities. Several methods of distributing the plastic material to the various cavities have been developed.
In one general type of distribution system, plastic material is fed into a single plastic entry port and then allowed to flow through runners or channels to the various cavities. In another distribution system, plastic material is fed into a single plastic entry port and allowed to flow through distribution passages to separate nozzles for injection into the individual cavities. This latter system is generally referred to as a "hot-manifold" system or mold. The hot-manifold system generally uses cartridge heaters within the steel block containing the distribution system and heaters associated with the nozzles to keep the plastic material in a molten state.
Both of these systems, while allowing the production of multiple plastic articles in a single molding cycle, suffer from serious drawbacks. Article cavities far removed from the single plastic entry point can easily receive too little plastic material, while those near the entry point can receive too much. In non-gas-assisted injection molding this problem can be at least partially solved by simply packing all the cavities with the maximum amount of plastic material. But very high plastic pressures are required to insure that the cavities furthest removed are fully packed. Such high pressures can create mold design and maintenance problems as well as higher operating costs.
And, of course, such a full-packing approach cannot be used in gas-assisted injection molding. Furthermore, the problems associated with unequal plastic distribution are even more apparent in gas-assisted injection molding. If a cavity receives too little plastic material, the resulting article may have thin wall areas or even holes that render the article useless. And if a cavity receives too much plastic material, significant increases in weight may result because of increased wall thickness.
Attempts to overcome these problems have involved the careful design and construction of the distribution channels within the molds to allow more even cavity loading independent of the location of the cavity. For example, cavities close to the plastic entry point could have relatively small diameter runners supplying plastic material whereas cavities far removed from the plastic entry point could be fed by larger diameter runners. In addition to significant design and cost problems, this approach has proven to be less successful than desired, especially as the number of cavities and the complexity of the required distribution system increases. In another approach, cavities are arranged so that each cavity is equidistant from the central plastic entry port. For example, the cavities could form a circle around the entry port. Such an approach significantly limits the number of cavities that in such multiple cavity tools.
In U.S. Pat. No. 4,279,582, a method is disclosed to individually control the amount of plastic in each cavity by the independent closing and opening of individual cavity gates in a non-gas-assisted injection molding process. Each gate is opened for a predetermined length of time to allow the cavities to be filled with the appropriate amount of plastic. At the end of the predetermined time, which could vary for each cavity, the individual gates are closed, thereby stopping the flow of plastic into that cavity. Each gate is controlled by a double action fluid motor and can be independently adjusted to vary size of the gate opening. Although this system represented an improvement in the art, it still retains significant limitations and problems. Variations in the plastic used, injection pressure, temperature, viscosity, flow rates, or other operational variables could result in each cavity receiving either too much or too little plastic material. In addition, the determination of the appropriate time for each gate to remain open and the appropriate size of the gate opening presented significant practical problems. Any changes in the opening and closing regime or in the gate dimensions for one cavity would likely affect plastic loading for other cavities. As the number of cavities or the complexity of the molded articles increases, the drawbacks and limitations of such a gate control approach becomes even more apparent. Furthermore, closing the gates does not provide positive shut off of the plastic flow. Incomplete gate closing is especially troublesome in gas-assisted injection molding. Therefore, the procedures of U.S. Pat. No. 4,279,582 are best employed with relatively simple molds having relatively few cavities and with non-gas-assisted injection molding.
Thus, the injection molding art still needs an effective and reproducible distribution system and method for filling and controlling the plastic loading of multiple cavities. The present invention, as detailed below, provides such a system and method to individually control and meter plastic material into article cavities in a multiple cavity injection mold.